An embodiment of the present invention relates to a method for the manufacture of an X-ray tube cathode filament. More specifically, an embodiment of the invention relates to a method that can be used to obtain a single-piece cathode filament. An embodiment of the invention also relates to an X-ray tube provided with a cathode filament of this kind.
An embodiment of the invention can be applied to X-ray tubes and particularly to tubes used in mammography, in devices used to study the vascular system or in scanners. In medicine, the device used generally as a cathode that emits electron beams is a cathode filament. At least one anode is positioned facing the cathode filament. The electrons emitted by the cathode filament strike the anode at high speed. The anode then emits X-rays.
For use in medicine, X-ray production requires great precision in the positioning of the cathode relative to the anode. Variations of more than 10 micrometers in the position of one of these elements relative to its expected position can have a deleterious effect on the strict control of X-ray production. During X-ray production, the cathode filament reaches a temperature of about 2800 degrees Celsius. The cathode filament therefore undergoes expansion. The expansion of the cathode filament may cause said cathode filament to shift in relation to the anode. This expansion can cause a break in the filament.
There is a known cathode filament comprising three parts: a filament body that is carried by two legs. The body of the filament emits electrons. The two legs of the filament are mutually parallel and perpendicular to the body of the filament. The legs are respectively soldered to two opposite ends of the body. Not only does the soldering method entail a delicate operation but it also causes the cathode filament to become brittle at the position of these solder zones. There is a risk that the filament will break at the position of these solder zones during an expansion.
To resolve the problem of mechanical embrittlement between the body and the legs of the filament, there is a known way of using a single-piece cathode filament. This filament is made out of the single plate curved in a U shape. Thus the two legs in the body forming the filament are made in one piece. The soldering step is eliminated.
The single-piece filament obtained is mechanically robust. However, the thickness of the legs is identical to that of the body. The rigidity of the filament obtained is therefore great. During the use of the X-ray tube provided with a cathode filament of this kind, the body of the cathode filament is subjected to expansion to a greater degree than are the legs. The mechanical resistance of the body is diminished, causing it to undergo shifts. The body of the filament has a length that increases owing to this expansion. Since the legs undergo less expansion, they have great rigidity and prevent the body of the filament from stretching. The body of the filament is therefore subjected to plastic deformation to the extent of getting curved. The positioning of the cathode relative to the anode is therefore modified in relation to the initial positioning. Once deformed, the filament body emits electrons in every direction. In medical engineering, it is often desired that the electron-emitting surface should remain perpendicular to the anode facing it. If the body is deformed uncontrollably, the filament can no longer be used.
The prior art cathode filaments are therefore not satisfactory. A filament having its body soldered to two legs risks breakage at the position of the solder zones when the filament undergoes expansion. There is a risk that the single-piece filament will get deformed during expansion, modifying the anode-cathode distance. This is incompatible with efficient operation of the X-ray tube that contains it.